Many known discharge lamps produce light by ionizing a vaporous fill material, such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel that maintains the pressure of the energized fill material and allows the emitted light to pass through it. The ionizable fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
High Intensity Discharge (HID) lamps are high-efficiency lamps that can generate large amounts of light from a relatively small source. These lamps are widely used in many applications, including highway and road lighting, lighting of large venues such as sports stadiums, floodlighting of buildings, shops, industrial buildings, and projectors, to name but a few. The term “HID lamp” is used to denote different kinds of lamps. These include mercury vapor lamps, metal halide lamps, and sodium lamps. HID lamps differ from other lamps because their functioning environment requires operation at high temperature and high pressure over a prolonged period of time. Ceramic discharge chambers for HID lamps have been developed to operate at higher temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. Such lamps with ceramic discharge chambers have been termed “CMH HID” lamps. Metal halide (e.g., CMH) lamps are widely used because they may have a higher efficiency than incandescent lamps. This is economically and environmentally beneficial.
These lamps, however, may sometimes experience reduced light output with time due to darkening of the inside of the discharge chamber walls. This darkening is due to tungsten being transported from the tip of the electrode during operation to the inside wall, blocking light. It has been proposed to introduce a calcium oxide or tungsten oxide dispenser in the discharge vessel, as disclosed, for example in WO 99/53522 and WO99/53523. This has been also achieved in an exemplary embodiment with improved lumen maintenance in U.S. Pat. No. 7,868,553 and US Patent Publication 2010/0013417, each of which has a common assignee as the present disclosure.
Despite the superlative lumen maintenance shown by many of the lamps described in the above-noted commonly assigned patent and patent publication, there is generally a desire for even higher efficacy and longer life for CMH lamp, with reduced wall and seal corrosion.